A prime consideration in the thermal protection of subjects exposed to extremely high temperatures, such as for example, aerospace vehicles or firemen's suits and tools, is the requirement for highly efficient lightweight thermal insulations. Aerospace vehicles, in particular, are subject to aeroconvective and radiative heating, launch acoustics and rain during atmospheric entry and consequently require extremely efficient thermal protection systems capable of protecting the metal or composite substructures from reaching temperatures above their operating limit.
Even though existing silica fibrous insulations, such as the Space Shuttle Orbiter's Advanced Flexible Reusable Surface Insulation (AFRSI), provide an excellent thermal protection for large portions of the Orbiter, as reported previously in AIAA; 82-0630 (May 1982) or Ceramic Eng. Sci. Proc., 6:793 (1985), there is a need to develop more efficient lightweight high temperature flexible insulations.
AFRSI is a quilt-like material made of two layers of silica and glass cloth with one layer of fibrous silica felt between them. The quilt is sewn together with silica and glass threads and has thickness between 1-5 cm. It has a temperature capability in excess of 650.degree. C. The AFRSI external cover is a silica fabric. A fiberglass is used on the internal surface which is then bonded to the shuttle skin with silicone adhesive. The AFRSI quilt is treated with a silane water repellant so that it will remain waterproof prior to launch. It has a density of approximately 9-10 lb/ft.sup.3, depending on its thickness.
This type of insulation evolved into another insulation called Tailorable Advanced Blanket Insulation (TABI), which is described in NASA CP:3001: 135-152 (November, 1987). TABI has a higher temperature capability and greater tolerance to aerodynamic loads than AFRSI. Although these insulations are progressively more durable, they all have comparable thermal conductivities of the order of 5.times.10.sup.-2 W/meter..degree. Kelvin (m.K) at room temperature at pressure of 0.01 atmosphere. It does not seem likely that this value could be reduced significantly with conventional materials. Thus, the main disadvantage of the prior art is its limitation to possess lower thermal conductivity which may be a critical factor for aerospace vehicles exposed to high aeroconvective thermal environments and which require a lightweight insulation.
However, certain improvement has been achieved with multilayer insulation (MLI) blanket, which has a much lower thermal conductivity. This MLI blanket is commonly used for thermal control within spacecraft. When the MLI blanket is combined with an all ceramic fibrous insulation such as the AFRSI or TABI, much lower effective thermal conductivities can be achieved.
A very lightweight MLI has commonly been used for spacecraft thermal control. For example, in 1982, NASA C. P., 2229:101-111 (May 1982), described a passive design for the thermal control of the Galileo Entry Probe. The design utilized radio-isotope heater units, multilayer insulation (MLI) blankets and a thermal radiator. The MLI blanket had 2 mm thick VDA Kapton.RTM. inner and outer layers and 11 layers of Dacron mesh alternated with 10 layers of 0.006 mm VDA Mylar.RTM., all these layers being electrostatically grounded to local conductive shield. The MLI's function for the Galileo probe was essentially to provide a low temperature insulation with primary aim to maintain certain temperature within the probe rather than having to function as insulator against outside heat.
A significant advance in thermal insulation for cryogenic applications was the development of multilayer insulation technology described previously in Advances in Cryogenic Engineering, 5:209 (1959), Plenum Press, and AFFDL TR-68-75 (April 1968). Cryogenic insulation technology is an important consideration for hypervelocity cruise vehicles which involve long-term propellant storage requirements. However, this type of insulation has been studied primarily at lower temperatures, and relatively little data and little practical experience have been reported for insulations in the temperature range of 900.degree. C. to 1100.degree. C.
It would, therefore, be advantageous to have available an insulation having a temperature capability of AFRSI around and above 1000.degree. C. and the lightweight of MLI which would be suitable for repeated use at these temperatures.
Recently, an insulation was developed which combined AFRSI and MLI into a single blanket. These multilayer insulations operating from 500.degree. C. to 1000.degree. C. temperatures were described in J. Fire Sci., 6:313 (1988). The insulations described consisted of ceramic aluminoborosilicate fabrics used for top and bottom surfaces, silica, aluminoborosilicate (ABS) or alumina insulations, and multilayers of stainless steel foils separated by ABS scrim, all quilted together using ceramic thread. The multi-layer configurations were made up of multiple stainless steel foil radiation shields layers separated by ABS scrim cloth layers. Fibrous ceramic insulation was laid on top of the multifoil configuration and the whole assembly was quilted in a lightweight ABS fabric and sewn with ABS thread.
While this type of insulation seems to be adequate and suitable for insulation at temperatures up to 800.degree. C., the oxidation of stainless steel occurring after multiple heating and cooling cycles effect negatively the spectral reflectance, and therefore made this insulation quite impractical for use in situations where the durability for repeated use at high temperatures is of importance.
Thus, it would be advantageous to have available a lightweight insulation having the thermal insulating capability for up to 2000.degree. C. suitable to be subjected to repeated heating and cooling.
From the above description of problems connected with design and fabrication of the insulation blankets suitable for high temperature insulations, the importance of the thread used for quilting of the blanket is apparent.
As pointed out above, AFRSI quilted ceramic blankets functioning as part of the thermal protection system (TPS) for the Space Shuttle Orbiter vehicle is an assembly of ceramic materials consisting of a fibrous silica batting sandwiched between a silica fabric and a glass fabric, sewn together in a 2.54 cm by 2.54 cm stitch pattern with a polytetrafluoroethylene (PTFE) sized silica thread. Since AFRSI provides thermal protection in temperatures only up to 650.degree. C., the thermal design requirements for advanced spacecraft exceed the temperature limits of the silica-type AFRSI used. For example, the Aeroassisted Space Transfer Vehicle (ASTV) will produce temperatures greater than 650.degree. C. as a result of an aeromaneuvering technique used to reenter low Earth orbit and the National Aerospace Plane (NASP) is also expected to generate high temperatures, in some cases above 1100.degree. C., as a result of hypersonic flight speeds.
Thus, there is a need for other ceramic materials suitable to be used at higher temperatures than 750.degree. C. As one of these materials, the high temperature resistant threads are very desirable for fabrication of these ceramic insulation blankets.
Previous studies desecribed in NASA C.P., 3001:135-152 (1988) have shown that silicon carbide (SIC) fabric surfaces can survive higher heating loads than silica fabric. For example, exposure to a heating rate of 37 W/cm.sup.2 resulted in no observable deterioration of the SiC fabric surface, whereas a silica fabric became brittle after exposure to 10 W/cm.sup.2. The higher emissivity of SiC over silica was found to be another advantage for high-temperature environments.
Thus, a high-temperature sewing thread of SiC yarn would be very desirable. This SiC thread would have to possess the capability of being machine-sewn into a quilted blanket using a construction process similar to that used for AFRSI. The outer mold line (OML) fabric and OML sewing thread used would be made of SiC instead of silica.
It is, therefore, an object of this invention to provide a lightweight, durable, flexible, reusable blanket-like multilayer insulation quilted with high temperature-resistant threads, such as SiC, zirconia, silicon nitride, TYRANNO FIBER.RTM. (composed of silicon-titanium-carbon-oxygen derived from organometallic precuror), and the like, which have the thermal insulation capability well above until now used insulations for use in the spacecraft industry, for furnace curtains, fire tools and equipment or for any other use where the temperature capability, the insulation efficiency and weight are all important.
Advantages of such insulation are partially described in AIAA-89-paper 1772 (June 1989) and in paper entitled "Composite Multilayer Insulations for Thermal Insulation," presented at Clemson University's Third Annual Conference on Protective Clothing on May 23-25, 1989. Development of SiC sewing thread is described in SAMPE Quart., 3-8 (1989).
The U.S. Pat. Nos. 3,007,596, 3,018,016, 3,152,003 and 3,274,788 are of general interest:
All of the references, patents, articles, standards, etc., cited herein are expressly incorporated by reference.